NASA has started updating plans and schedules for additional SLS Block 1 launches in the early 2020s after Washington added federal budget money for a second Mobile Launcher (ML) platform and umbilical tower in late March.
Construction of a new Mobile Launcher frees the first ML from a three-year long downtime for teardown and reassembly after the first SLS launch of Exploration Mission-1 (EM-1), currently projected for mid-2020. Instead of being retired after one launch, the Block 1 configuration could fly multiple times.

The first two candidates to fly on the second SLS launch are the first crewed Orion on a Lunar swingby flight test or the Europa Clipper interplanetary flagship that will repeatedly overfly Jupiter’s moon exploring for signs of habitability. The space agency formally re-assigned both missions to Block 1 last week and will decide in the future which one gets priority.

Both missions will need Block 1’s Interim Cryogenic Propulsion Stage (ICPS), a modified Delta 4 upper stage, and NASA is now looking at purchasing more stages and human-rating the design for Orion missions.

ML-2 implications for SLS Block 1

Nearly halfway through Fiscal Year 2018 (FY 2018), Congress passed a detailed appropriations bill in late March which was then signed into law by President Trump. Absent from previous budget drafts, the bill somewhat suddenly added $350 million to the top-line of the Exploration Ground Systems (EGS) program budget “for a second mobile launch platform and associated SLS activities.”

Enactment of the detailed budget came over a month after the Trump Administration had published its proposal for next year’s FY 2019 budget, and given the costs involved they chose to spend the money elsewhere and not build another ML. Instead, they stuck with the earlier plan to utilize one Mobile Launcher, which is being modified from its original Ares I structure and outfitted to service the SLS vehicles with the bigger set of supplies they need to launch.

The Mobile Launcher includes both the platform that the rocket stands on and an umbilical tower that connects the different elements of the launch vehicle to ground supplies and provides additional structural support.

SLS will debut with an uncrewed Orion spacecraft on EM-1 in its Block 1 configuration, which has an ICPS upper stage stacked on top of its foundational elements of two five-segment Solid Rocket Boosters (SRB) and a four-engine, liquid-propellant Core Stage.

Diagram of SLS Block 1 Crew elements. Credit: NASA

NASA recently published an updated performance figure, now saying the Block 1 configuration is capable of lifting over 95 metric tons into a circular reference orbit of perhaps 100 nautical miles.

Rather than consume all the SRB, Core Stage, and upper stage performance to deploy a payload in Low Earth Orbit (LEO), on EM-1 the Boosters and Core Stage by themselves will put Orion and the ICPS in an elliptical Earth orbit of 975 by 22 nautical miles around eight minutes after liftoff. The low, suborbital perigee allows for a safe, ballistic ocean disposal of the now empty Core Stage.

The 975 nautical mile apogee imparts as much performance as possible from the Core Stage to the Orion and ICPS. When they reach apogee, the upper stage makes a short burn to bring the perigee up to orbital altitude, resulting in a 975 by 100 nautical miles.

Close to perigee, the ICPS will make a long Trans Lunar Injection (TLI) burn to send Orion on its test flight to orbit the Moon and return.

The prior plan was to retire Block 1 and move to the larger, more capable Block 1B configuration immediately after EM-1. That configuration will use a larger, higher-performance Exploration Upper Stage (EUS) that is still under development.

The EUS is longer than ICPS, and the single Mobile Launcher was going to require a significant teardown/disassembly and reconstruction to support the longer, EUS-based Block 1B vehicle, along with structural reinforcements to handle the higher weight of both the vehicle and the modified umbilical tower.

Side-by-side comparison from 2016 of the Block 1 vehicle and ML umbilical tower configuration (left) and the Block 1B configuration. Prior to funding ML-2, the single Mobile Launcher was going to be partially taken apart and rebuilt to go from the left configuration to the right. Subsequent design iterations for the Block 1B tower have changed the EUS umbilicals and added a Vertical Stabilizer Damper above. Credit: NASA

It was expected that the program’s only Mobile Launcher would be out of service for nearly three years, a 33-month long period that came to be referred to as an “iron-bar” stuck in the middle of scheduling charts that could not be shortened.

“After I fly EM-1, the mobile launcher I have today I would then have to start modifying for the Exploration Upper Stage,” NASA Acting Administrator Robert Lightfoot said in testimony at a House of Representatives subcommittee hearing on April 12. “So while I’m doing that modification, I can’t fly. I’m just down and there’s a 33-month time period there.”

With the first SLS launch of EM-1 now projected in mid-2020, the second could not be any sooner than some time in 2023.

Although a second ML was looked at in the past at a high level, the subject came up more frequently last year during NASA advisory group meetings. The Aerospace Safety Advisory Panel (ASAP), which reports to NASA and Congress, recommended construction of another ML in its 2017 annual report, following discussions in the last three of its quarterly meetings of the year.

“This creates potential safety risks as the skills and number of the ground and launch workforce may naturally attrit over such a long inactive period, resulting in a critical loss of experience and knowledge,” ASAP wrote in their report regarding the modification downtime. “The ASAP strongly recommends that NASA be resourced and begin construction of a second MLP as soon as possible.”

Now with initial payment for what will become ML-2, reconstruction plans for ML-1 have been deferred indefinitely to focus on a ground-up build of ML-2.

Instead of being out of service for three years, ML-1 will remain in its Block 1 configuration after EM-1. It is expected that following post-launch inspections and refurbishment, ML-1 would essentially be ready to support preparations for another Block 1 launch.

“We knew [EUS] wasn’t necessarily required for every job the SLS might have,” Dr. Donald McErlean noted during the ASAP’s second quarterly meeting May 17 at the Kennedy Space Center in Florida.

Block 1B flights now won’t begin flying until EUS and ML-2 are ready. ML-2 was initially projected to take around five years to complete following a future contract award; however, the House of Representatives Appropriations Committee has weighed in again and further specified their expectations.

A report submitted by Representative John Culberson of Texas to accompany the House’s FY 2019 appropriations bill which covers the Commerce, Justice, Science and Related Agencies (CJS) says in part: “The fiscal year 2018 Appropriations Act included $350,000,000 to begin procuring a second mobile launch platform (MLP) and associated infrastructure to launch Exploration Mission-2 earlier than currently envisioned,” echoing report language that accompanied the enacted FY 2018 bill.

The recent draft report for the CJS bill, which covers NASA’s funding, was published on May 16.

Above and beyond the FY 2018 report language, the draft report added: “NASA shall expeditiously implement processes necessary to construct the second MLP so it is completed by 2022. A second mobile launch platform dramatically changes the mission profile for NASA and will allow NASA to launch human and robotic missions at a cadence to ensure the Lunar Orbital Platform is built more quickly and to ensure that the Europa Clipper and Lander missions are not delayed by the overall launch cadence profile.”

NASA is only beginning the formal procurement process for ML-2, with an initial Request for Information (RFI) posted on May 11 noting a performance period of 44 months. Although the House report is only a draft, it indicates interest with those appropriators in Congress to have the contract awarded in time for the 44-month period to fit between now and the end of 2022.

The committee also noted in the report their understanding that the EUS Critical Design Review (CDR) was scheduled to occur this summer.